Optimizing gamma radiation shielding with cobalt-titania hybrid nanomaterials

Cobalt-doped titania nanocomposites were fabricated to be utilized for radiation shielding aims. The chemical composition of the composites was measured using the energy-dispersive X-ray spectrometer. Moreover, the structure of the composites was evaluated using the X-ray diffractometer, and the morphology of the fabricated composites was presented using the scanning electron microscope. Furthermore, the γ-ray shielding properties were estimated using the Monte Carlo simulation between 0.059 and 2.506 MeV. The linear attenuation coefficient of the fabricated composites decreased by factors of 93% for all samples by raising the incident γ-energy between 0.059 and 2.506 MeV. Moreover, the partial replacement of the Ti4+ by Co3+ slightly enhanced the linear attenuation coefficient from 0.607 to 0.630 cm−1 when the Co3+ increased from 0 to 3.7 wt%. The improvement in the linear attenuation coefficient causes an enhancement in other radiation shielding properties.

www.nature.com/scientificreports/ TiO 2 , visible light TiO 2 photocatalysts are created, which can be used to remove water pollutants. Cobalt oxides are one of these dopants that are drawing attention because of their god speed in the photocatalytic-reduction of Carbon dioxide (CO 2 ) in visible and ultravilot light and dye-sensitized solar cells for energy production 18,19 .
To understand the shielding abilities of certain materials, radiation shielding parameters need to be calculated and analyzed across a wide range of energies. Monte Carlo simulations are used to determine these values such as linear attenuation coefficients using a specific set of conditions. These simulations can be used to understand the properties of material before experimentally determining these parameters, to save time and resources, or alongside theoretical calculations such as XCOM to ensure the two methods agree with each other. After the initial values are obtained, further parameters can be determined to gain a full understanding of the radiation shielding abilities of the material [20][21][22][23][24][25] .
The concept presented in present study is estsblished on the exact planning of a two dimintion (2D) nanoparticular structure of cobalt oxides and titanium oxides linked by carbon sheets. The inclusion of cobalt contributes to thermal stability, corrosion resistance, and wear, which makes it, along with titanium, useful in many industries. Additionally, cobalt oxides stand out due to their capacity for coloring. The two-dimensional carbon sheets increase the surface area of the prepared nanomaterial. The newly synthesized compounds' characterization and γ-ray protection particulares has been inspected.

Methods and materials
Manufacture of materials. (I) A known size and concentration (0.4 mL-0.1 M) of hydrous potassium chloride, 98 mL solution of ethanol, and 1.6 mL of (Ti 4 O 28 H 12 C) titanium isopropoxide have been mixed together and agitated for 5 h. All the contents were then moved to a Teflon-lined stainless steel autoclave with a 100 mL capacity.Then the temperature was raised to 170 °C for 35 h. After that, the autoclave has been brought to cooling naturally at room temperature. The finished product was gathered thoroughly and soaked in ethanol and has been dried in the oven at 70 °C after filtering. Finally, the solids were transferred to the furnace to be calcined at a temperature of 500 °C for 4 h to produce TiO 2 NPs. (II) to produce a Cobalt-doped TiO 2 nanocomposite (Co-TiO 2 ), 0.9 gm of cobalt acetate is added to the mixed solution before transferring it to the autoclave and then following the same previous steps. (III) The method for preparing cobalt-doped TiO 2 loaded on carbon sheets (Co-TiO 2 /C) involves mixing 0.5 g of Co-TiO 2 nanocomposites produced in step (ii) with 80 mL of an aqueous solution containing 0.02 g glucose. The mixture has been stirred for 4 h, then placed in the autoclave used in the previous steps then heated for 30 h at 170 °C. After that, leave the autoclave to cool naturally in the air. The solids were gathered thoroughly and stirred well in water for washing and have been arid in the furnace at 70 °C. Finally, the solids powder have been calcined for 4 h at 500 °C.
Characterization of the fabricated materials. Utilizing a (PANalytical X'Pert Pro diffractometer, Netherlands), XRD patterns have been obtained. To comprehend the crystallographic phase, a monochromatic CuKa radiation source with a wavelength of (0.15406 nm) was set to (45 kV) and (40 mA) at 25 °C. The diffractograms were captured with a step size of (0.013) over a scan range of 20°-80°. To predestine the morphology of the fabricated solid powders materials and appreciation the element analysis, scanning electron (SEM) microscope and an energy-dispersive X-ray spectrometer (EDX) havebeen used (JEOL JSM-6400-Jeol Ltd., Tokyo, Japan). The TEM analysis, which was performed using a (Philips CM200, America) transmission electron microscope with a 200 kV accelerating voltage, provided information about the morphological structure of the composite. The crystallinity of various components in the produced composite was determined using Raman spectroscopy with a 532 nm laser excitation (GL Gem Raman™ PL532, Canada). γ-ray shielding capacity estimation. Utilizing the MCNP5 code, the ability for γ-ray shielding for the newly synthesized cobalt-doped titania nanocomposites was simulated in the γ-ray energy ambit of (0.059-2.506 MeV). The MCNP code utilized the nuclear database ENDF/B.VI.8 and the tally F4 to estimate the median flux per unit cell and average track length (ATL) of gamma photons in the chosen composites. To estimate the ATL of gamma photons in the manufactured materials an input file contains all information about the fabricated nanocomposites, radioactive source, detector, and tally, as illustrated in Fig. 1. The required information was introduced to the MCNP's input file through some definite cards such as cell, surface, materials, source, and tally cards. After runout the simulation process, a new output file was created and has a txt format. The created output file contains all required information about the ATL, the number of collisions in the material cell as well as the relative error in the simulation process. The relative error recorded for all samples ranged from ± 3%. Detailed information about created geometry and the dimensions of the geometry components collimators, sample, detector, and source was discussed in some previous works 26,27 .
Using some mathematical equations, the simulated MCNP's ATL for the fabricated composites was transferred to the linear (µ) and mass (µ m , cm 2 /g) attenuation coefficients. Then, based on the estimated µ, the half-value thickness (Δ 0.5 ), lead equivalent thickness (Δ eq ), transmission factor (TF), and radiation protection efficiency (RPE) were evaluated as illustrated in the following equations 28 .
N 0 , N t , and x are the number of emitted photons from the radioactive source, the transmitted photon number through the fabricated composite with a definite thickness (x, cm).   29 . The XRD results of TiO 2 doped by Co and C nanocomposites demonstrate the lack of any free cobalt phases, which implies that all the cobalt is a fuse in anatase crystallites. XRD results show that there is a lack of obvious carbon peaks, indicating that the carbon in the composite has a low degree of crystallinity. Surprisingly, the two samples' Co-TiO 2 and Co-TiO 2 /C diffraction patterns resembled those of virgin TiO 2 , with a minor change in the position of the peak. The slight shift in peak position in the XRD pattern is the result of the modification in structure at the local level around Ti 4+ resulting from the substitution of Co 3+ and Co 2+ . This suggests that Cobalt dissolved within the lattice. Another notable change with the inclusion of cobalt is the change in the color of TiO 2 . All of the XRD patterns revealing that the average size of the nanocrystalline particles of pure TiO 2 changed when cobalt or cobalt/ carbon is added. According to Scherrer's equation (Fig. 3) 29 , the average crystallite size of prepared samples: pure TiO 2 (TiO 2 nanoparticles), single doped TiO 2 (Co-TiO 2 nanocomposite), and double doped TiO 2 (Co-TiO 2 /C nanocomposite) were 11.7, 12.4, and 13.24 nm, respectively. The samples' increased crystal size emphasizes that cobalt is replacing Titanium in the lattice of TiO 2 (anatase phase). An examination of the main peak in XRD diffraction (101) in Fig. 2 (inset) when magnified, shows a significant shift towards higher 1 theta values when www.nature.com/scientificreports/ cobalt and carbon are introduced as dopants. Additionally, the average crystallite size of double-doped TiO 2 by C and Co (Co-TiO 2 /C) material is found to be greater in comparison to that of the single-doped TiO 2 by Co only (Co-TiO 2 ). These findings suggest that there is a synergistic effect between carbon and cobalt when they are combined together. The XRD results reveal that the average crystallite size of the two manufactured materials doped by (Co) and (Co, C) are almost identical. Furthermore, their positions of diffraction peaks are similar, which implies that carbon doping has a minor impact on the crystal structure 30,31 .
Raman shifts analysis. Figure 4 shows the Raman spectrum of three fabricated samples in the 50-2000 cm −1 range. The Peaks which apperant at 151.5, 391.5, 513, and 631.5 cm −1 were assigned to TiO 2 (anatase crystalline phase) modes Eg, B1g, A1g, and Eg, respectively 32 . All peaks show a significant decrease in intensity after cobalt doping, along with some shift in peak position. Although these findings imply that the anatase phase was not entirely altered by the Co, they also reveal a significant long-order crystallite distortion, which agrees with the XRD findings. The substitution of Ti 4+ by Co 3+ and the disparity in their cationic charges could explain this distortion. Furthermore, doping by Co and C also correlated to the formation of vacancies in the oxygen chain, which significantly affects the vibration of the bond between Ti and O (Ti-O). It is substantial to point out while the size of the nanoscale crystalline phase, which influences the frequency of shifting brought on by phonon confinement, limits the peak position shift. The A1g peak shows a clear shift in location and an obvious rise in intensity. Modification in the A1g peak indicates the formation of more vacancies in the oxygen chain, mainly likely as a consequence of the reaction with carbon in the hydrothermal reaction. According to the strong Raman peak at 689 cm −1 , the anatase Eg modes shifted as a result of being doped by carbon and cobalt 33 . Even after the calcination of the material containing carbon in the air, the existence of carbon can be established by seeing  SEM and TEM analysis. Figure 5a-c shows SEM of TiO 2 nanoparticles, TiO 2 doped by Co (Co-TiO 2 ), and when adding carbon (Co-TiO 2 /C) after 500 °C calcination. The results show that nanoparticle clusters form in all samples. Surprisingly, the two samples of TiO 2 which were doped by (Co) and (Co, C) form 2D sheets. Undoped TiO 2 nanoparticles, on the other hand, tend to form more irregular shapes when clustered. After the addition of cobalt and carbon, the appearance of distinct nanosheets indicates a predilection for development in particular directions. After the addition of cobalt, there is most likely a rise in state density perpendicular to the C-axes; in addition, partial oxygen removal occurs due to the presence of carbon. The 2D sheets are only weakly attached to one another to create a zigzag pattern with pores, which is seen in Fig. 5b,c. The TEM images ( Fig. 5d-f) provide additional information concerning fabricated materials' morphology. According to the find-  www.nature.com/scientificreports/ ings, each of the three types of material put to the test is made up of nanoparticles that have parallel sides and measure approximately 23 nm in size on average. At the edges, the particles start fusing together after adding Co which forms 2D nanolayer sheets. Then, when glucose has been added as a carbon source, the clusters and average particle size were increased without any distortion in the anatase phase. The EDX spectra and element mapping (Fig. 6) show pure TiO 2 , Fig. 7 indicates the existence of (Co) element in the single doped prepared sample (Co-TiO 2 ), and Fig. 8 indicates the existence of (Co, C) elements in the double doped prepared sample (Co-TiO 2 /C). There were no peaks associated with Co or C in the XRD patterns. As a result, the Co ions in the single-doped case (Co-TiO 2 ) and (Co, C) ions in the double-doped case (Co-TiO 2 /C) are distributed among the titanium dioxide crystallites (anatase phase) in a uniform form. In Table 1, the elemental analysis composition of TiO 2 nanoparticles after doping by Co and C can be determined.
Radiation shielding properties. The µ m and µ are the main important parameters that describe the capacity of newly synthesized cobalt-doped titania nanosheets on resisting the transport of photons. Both µ m and µ were evaluated for the fabricated nanocomposites (pure TiO 2 nanoparticles, single doped TiO 2 (Co-TiO 2 ) nanocomposite, and douple doped TiO 2 (Co-TiO 2 /C) nanocomposite) using the MCNP code as well as the XCOM software program over a wide γ-ray spectrum (E γ , MeV) ranging from 0.059 to 2.506 MeV. The µ m behavior is depicted in Fig. 9 Figure 9b shows a decrease in the µ m values between 0.0989-0.0618 cm 2 /g for the TiO 2 NPs, between 0.0991 and 0.0619 cm 2 /g for the Co-TiO 2 composite, and between 0.0991 and 0.0619 cm 2 /g for Co-TiO 2 /C with raising the E γ values between 0.344 and 0.964 MeV. The presented reduction behavior was studied using the cross-section of the Compton scattering, which varies   Figure 9c depicts a slight reduce in the µ m values associated with any increase in the E γ values within the energy interval 1.173-2.506 MeV. The illustrated slight reduction is due to the fact that the CS interaction is still the predominant interaction also between 1.173 and 2.506 MeV. Therefore, the PE, and CS interactions only appeared during the result analyses while the pair production interaction (PP) was not observed in the studied energy range. The delayed appearance of PP interaction in the current work is attributed to the relatively small density of the manufactured composites. The PP interaction in the current work starts at gamma-ray energy higher than 3 MeV where the high variation in the gamma energy (in the PP interval) causes a very slight increase in the µ m values. This behavior is attributed to the proportionality of the PP cross-section with Log E. The simulated results using the MCNP were confirmed using the theoretical free program XCOM as illustrated in Fig. 9. The obtained results showed an agreement with the simulated data with a range between ± 2%. Also, the modification of the titanium dioxide (TiO 2 NPs) with Co and C nanoparticles has a slightly enhancing effect on the µ m values. Figure 10 35 . Then decreasing the concentration of Ti nanoparticles in the manufactured nanocomposites was associated with an increase in the µ m values at low γ-photon energies while the mentioned trend was reversed for the high γ-photon energy. Because of the low concentrations of Co and C nanoparticles added to the TiO 2 nanoparticles, the variation in simulated µ m values is relatively small. The Δ eq describes the synthesized composites' thickness which is able to attenuate the same photon number as a 1 cm thickness of the pure lead (Pb). Figure 11 depicts the change in the µ, Δ 0.5 , and Δ eq values versus E γ values for the fabricated composites [a] TiO 2 nanoparticles, [b] single doped TiO 2 (Co-TiO 2 ), and [c] double doped TiO 2 (Co-TiO 2 /C) versus. Figures 11 a, b, and c show a decrease in µ and Δ eq values and increase the Δ 0.5 values. The evaluated µ values decreased shapely between 1.884 and 0.127 cm −1 (for TiO 2 NPs), between 1.970-0.131 cm −1 (For Co-TiO 2 /C nanocomposite), and between 1.955 and 0.132 cm −1 (for Co-TiO 2 /C nanocomposite) with raising the E γ . The reduction of the µ was caused by the effect of PE, CS, and PP interactions. The highest calculated Δ eq values are 83.978, 80.772, and 80.977 cm for the fabricated pure TiO 2 nanoparticles, single doped TiO 2 Co-TiO 2 composite, and double doped TiO 2 Co-TiO 2 /C composite, respectively. The calculated Δ eq decrease rapidly as the E γ values increase due to the large decrease in the Pb's µ values compared to the decrease achieved in the fabricated composites' µ value. Between 0.059 and 0.122 MeV, the Δ eq increased by a factor of ≈ 103% for the fabricated composites. The mentioned increase in the Δ eq is attributed to the low drop in Pb's µ compared to the reported drop in the fabricated composites µ values. The Pb's µ droped by a factor of ≈ 34%, whereas the tested composites' µ values decreased by a factor of ≈ 67% when the E γ was raised between 0.059 and 0.122 MeV. After that, in the intermediate energy interval between 0.344 and 0.964 MeV, the Δ eq values were reduced by factors of ≈ 62%. This reduction is due to a large reduction in Pb's µ values compared to the moderate µ values reduction for the fabricated composites. In the CS interaction interval, the Pb's µ value droped by 76% and the µ for the synthesized composites decreased by 37.57, 37.60, and 37.57% for TiO 2 nanoparticles, single doped TiO 2 (Co-TiO 2 ), and double doped TiO 2 (Co-TiO 2 /C). Because the µ values for Pb and the fabricated composites are independent of the γ-photon energy, there is no significant variation in the Δ eq values with variation in the E γ values in the high E γ interval between 1.173-2.506 MeV. The Δ 0.5 values show an increase with raising the E γ values due to the reduction recorded for the µ values, where the Δ 0.5 values are inversely proportional to the µ (see Eq. 3).
The change in µ, Δ 0.5 , and Δ eq values vs. the amounts of Ti 4+ and Co 3+ nanoparticles at E γ of 0.662 MeV was given in Fig. 12. The µ values decreased with raising the Ti concentrations while they increased with raising the Co concentration. While, the Δ 0.5 , and Δ eq increased with raising the Ti concentrations while they decreased with raising the Co concentrations at E γ of 0.662 MeV. The Δ 0.5 values increased slightly from 2.730 to 2.827 cm with raising the Ti 4+ concentration from 55.4 to 61.9 wt% while it decreased with raising the Co concentration from 0 to 3.7%. Furthermore, the Δ eq values at the same E γ of 0.662 increase from 4.896 to 5.069 cm with raising the Ti nanoparticle concentration from 55.4 to 61.9 wt% while it decreases from 5.069 to 4.896 with raising the Co 3+ nanoparticle concentration from 0 to 3.7 wt%. The illustrated behaviors for µ, Δ 0.5 , and Δ eq were attributed to the partial replacement of Ti 4+ nanoparticles with Co 3+ nanoparticles where the partial replacement of the Ti by Co nanoparticles causes an increase in the electron density of the composites which creates more resistance for passing the incident photons. Figure 13 illustrates a comparison between the fabricated composites µ values and the µ values for some previously reported composites and common commercial shielding glasses at E γ of 0.662 MeV. The fabricated   The TF and RPE for the selected composites were exhibited in Fig. 14. A decrease in the TF values between 15.2 and 88.0% (for composite TiO 2 ), between 13.9 and 87.7% (for Co-TiO 2 composite), and between 14.2 and 87.8% (for Co-TiO 2 /C composite) when the E γ increased from 0.059 to 2.506 MeV. On the other hand, the RPE decreases from 84.8 to 12.0% (for composite TiO 2 ), from 86.1 to 12.3% (for composite Co-TiO 2 ), and from 85.8 to 12.3% (for composite Co-TiO 2 /C) when the E γ varied between 0.059 and 2.506 MeV. The previously discussed behaviors for both parameters are attributed to the penetration ability of the incident γ-photons, where raising the energy causes an increase in the penetration power of the photons due to the decrease of the photon wavelength. Therefore, the emitted photons (N o ) pass the tested composites with a low number of collisions with the surrounding electrons. Thus, the N t increase is associated with a decrease in the number of photons absorbed in the fabricated composites (N a ). The mentioned increase in the N t as well as the decrease in the N a photons give rise to the increase in the TF and decrease in the RPE with raising the γ-photon energies.
The dependence of TF and RPE values on the composites' thickness was illustrated in Fig. 15. The TF values decreased and the RPE increases for all fabricated composites when the composite thickness grew between 0.025 and 2 cm. When the thickness ranged between 0.25 and 2 cm, the TF values decreased linearly from 93.3 to 57.6% (for TiO 2 composite), between 86.7 and 56.6% (for Co-TiO 2 composite), and between 86.7 to 56.5% (for Co-TiO 2 /C composite). But, the RPE increased linearly from 6.7 to 42.4% (for TiO 2 composite), from 6.9 and 43.4% (for Co-TiO 2 composite), and from 6.9 to 43.5% (for Co-TiO 2 /C composite) when the thickness of the composite raised from 0.25 to 2 cm at a gamma photon energy of 0.511 MeV. Raising the composite thickness causes an increase in the path length of incident γ-photons inside the fabricated composites. Therefore, the possibility of the incident photons interacting with the surrounding electrons increases. Thus, the consumed energy by incident photons increased inside the material with higher thicknesses than the lower one, resulting in a significant increase in RPE values accompanied by a corresponding decrease in TF values 39 .

Conclusions
A Cobalt-doped TiO 2 nanocomposite with more vacancies in the oxygen chain was synthesized based on a hydrothermal method in one step and the formation and crystallinity of the synthesized composite were confirmed using XRD in 2D sheets with an average crystal size of less than 13 nm. Moreover, the µ m of the cobalt-doped titania nanocomposites was estimated using MCNP5 between 0.059 and 2.506 MeV. The simulated values of www.nature.com/scientificreports/ the mass attenuation coefficient were confirmed using the XCOM program, where the difference between the XCOM and MCNP results ranged between ≈ ± 2%. The mass attenuation coefficient decreased from 0.567 to 0.038 cm 2 /g (TiO 2 composite), 0.567 to 0.038 cm 2 /g (Co-TiO 2 composite), and from 0.570 to 0.038 cm 2 /g (Co-TiO 2 /C composite) in the studied energy region. The mass attenuation coefficient values increase with decreasing the Ti 4+ ions in the fabricated composites at low γ-photo energy while it has no considerable changes with substitution of Ti 4+ by Co 3+ ions at intermediate and high γ-photon energy. Furthermore, the µ is slightly increased by raising the Co 3+ ions between 0 and 3.7 wt% in the fabricated composites. The mentioned increase in the µ values was associated with a slight decrease in Δ 0.5 and Δ eq values. Based on the concluded results, the fabricated composites-based TiO 2 nanocomposites can be used as fillers to improve the gamma-ray shielding capacity of polymers, ceramics, and paint materials. The fabricated composites have a good shielding capacity in low and intermediate gamma-ray energy intervals but it is not suitable for high gamma-ray energy applications.

Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.